1. Field of the Invention
The present invention generally relates to color image forming devices that print an image according to image data and, more particularly, to a color image forming device that can correct a registration error of an image with high accuracy at reduced toner consumption.
2. Description of the Related Art
There is known a tandem-type color image forming device, which comprises a plurality of image forming units arranged around the photosensitive member in a direction of travel of a recording medium. Each of the image forming units includes a photosensitive member and charging means, exposing means and developing means arranged around the photosensitive member. In the color image forming device, toner images of different colors are formed on the photosensitive members and the color images are sequentially transferred onto a recording medium and the color toner images are fixed on the recording medium.
The tandem-type color image forming device can easily achieve a high-speed operation since the image forming unit is provided for each color. However, there is a problem that an image quality is deteriorated since it is difficult to form each color image on the recording medium without registration error between the color toner images. One of causes of such a registration error is a positional shift of the color toner image due to a relative registration error generated between the image forming units caused by a change with time or a change in temperature.
Moreover, there also is a registration error generated between the color toner images due a small error or fluctuation in a velocity of photosensitive member and an intermediate transfer member. In order to prevent the image quality from being deteriorated due to such a registration error, there is known a method of correcting a registration error by forming color toner images, that is, patch patterns, on the recording medium or the intermediate transfer member so as to correct the registration error by detecting the patch patterns by a photo-detector.
In order to suppress the above-mentioned registration error as small as possible, it is required to raise the detection accuracy of the registration error. Usually, it is preferable to set the registration error between the color images to 100 μm or less at maximum and 50 μm or less on the average. In order to do so, it is desired that the image position detector can detect a registration error with an accuracy of 10 μm or less.
A registration error of each color toner image relative to a reference color toner image is calculated using a detection signal acquired from the image position detector so as to perform a correction control of a registration error by adjusting a write timing of an optical unit in each image forming unit other than that of the reference color or perform a correction control for correcting a position of the optical units.
However, each time when a relative registration error is generated between the image forming units due to a temperature rise in the device after registration error correction is made by an image position detector, the patch patterns must be formed so as to detect and calculate an amount of registration error by the image position detector. Accordingly, there is a problem in that a throughput of printing is deteriorated and an amount of toner consumption is increased, which increases a page cost. Thus, there is suggested a correction control by predicting an amount of registration error by detecting a temperature.
The technique to carry out such a predictive control of a registration error in response to a temperature is disclosed in the following patent documents 1 and 2.
Patent Document 1: Japanese Laid-Open Patent Application No. 3-293679
Patent Document 2: Japanese Laid-Open Patent Application No. 2003-207976
A description will now be given, with reference to FIG. 1 through FIG. 3 of an example of a conventional predictive control of a registration error. FIG. 1 shows a process flow of a conventional predictive control method. First, in step 101, patch patterns are formed on a recording medium, and a correction of a registration error is carried out by detecting the patch patterns. Then, in step 102, a temperature inside the image forming device is detected, and the detected temperature is stored in a memory of a control device as a reference temperature. In this example, the reference temperature is set as T0.
In step 103, it is determined whether or not the temperature inside the device reaches a temperature of predictive registration error correction timing. Here, as shown in FIG. 3, a time when an absolute temperature inside the device reaches T1, T2 or T3 is determined as correction timing. It should be noted that the above-mentioned absolute temperature does not mean a physical absolute temperature, but means a temperature absolute to a relative temperature. Of course, the predictive correction is not necessarily performed when the temperature (absolute temperature) inside the device reaches T1, T2 or T3, but may be performed when a temperature difference (relative temperature) between the temperature inside the device and T0 reaches a predetermined value.
In this example, in order to perform the predictive registration error correcting operation, a correction table as shown in FIG. 2 is stored in a memory of the control device. The correction table indicates that a predictive registration error correction value is ΔE0, ΔE1, ΔE2, . . . when the temperature inside the device is an absolute temperature of T0, T1, T2, . . .
Then, if a result of the determination in step 103 is affirmative (YES), that is, for example, the reference temperature is T0 and if the temperature reaches T1 by a temperature rise after that, the routine proceeds to step 104 so as to perform an operation to correct the amount of registration error ΔE1 corresponding to the temperature T1. Similarly, if the temperature of the device reaches T2 or T3, the predictive registration error correction value ΔE2 and ΔE3 are read by referring to the table shown in FIG. 2 so as to perform a registration error correcting operation in accordance with the predictive registration error correction values.
Further, in step 105, it is determined whether or not an image registration error detection correcting operation timing is reached. That is, the timing is determined according to not the prediction but a result of detection of an actually generated registration error by actually forming patch patterns on the recording medium. The image registration error detection operation timing may be a time when a predetermined time period has passed or a time when the temperature inside the device exceeds a temperature risen from the reference temperature by a predetermined value. If a result of the determination in step 105 is affirmative (YES), the routine returns to step 101 so that the same operation is repeated.
However, if a registration error other than that defined in the table is generated, for example, if a change in an amount of registration error due to individual variation or a change in an amount of registration error due to long time of use is generated, there is a problem in the above-mentioned color image forming device that there is no means for automatically correcting the registration error, which results in an increase in an error of the predictive registration error correction according to a temperature and invites deterioration in the image quality, since the above-mentioned conventional example predicts an amount of registration error according to a relationship (table) between to a previously set temperature and an amount of correction so as to perform a correction.
On the other hand, in a case where the image forming units are arranged in a vertical direction, the image forming unit in the upper stage is given an influence of heat generated by the image forming unit of the lower stage, and there is a case where a temperature change rate per unit time of the image forming unit of the upper stage may be different from that of the image forming unit of the lower stage. Thus, there is a case where a single predictive registration error correction table cannot cover all cases.